1. Field of the Invention
The present invention generally relates to charged particle beam tools and, more particularly, to electron beam projection lithography tools having electron (charged particle) emitter or cathode of large dimensions.
2. Description of the Prior Art
Charged particle beam tools are known for numerous applications such as electron beam microscopy. Numerous applications have also been developed in the semiconductor device manufacturing industry such as for impurity implantation and lithographic exposures of patterns having feature sizes with dimensions smaller than can be resolved using electromagnetic radiation and the manufacture of x-ray lithography masks which must be formed at the same minimum feature size as the exposure to be made. In this regard, reduced feature sizes and increased integration density in integrated circuits provides increased performance and functionality as well as providing increases in manufacturing economy.
However, the graphic pattern complexity increases with integration density. The well-known probe-forming type of electron beam lithographic exposure tool is generally arranged to expose a small shaped spot repeatedly at high speed to form the desired pattern. The shaped spot may include only a relatively few (e.g. one hundred or fewer) pixels while the complete chip pattern may include millions of pixels for current designs and numbers of pixels orders of magnitude larger are foreseeable. Therefore, while probe-forming electron beam exposure tools are often used at the present time for custom chips and application specific integrated circuits (ASICS) which are usually manufactured in limited quantities, probe-forming tools have insufficient throughput to support production quantities of integrated circuits at the present state of the art.
Accordingly, electron beam projection tools have recently been developed. These tools project a sub-field of a desired pattern formed in a reticle onto the target such as a resist-coated wafer. The sub-fields, while small compared to the entire chip pattern, are large in comparison to the circuit feature sizes and may include tens of millions of pixels which may be simultaneously exposed at rates only slightly reduced, if at all, from the exposure rate of probe-forming beam tool exposures. Unfortunately, an electron beam projection tool must deliver images at the target or wafer plane of extremely good fidelity to the reticle pattern and are thus of substantial complexity in design and difficult to adjust in order to obtain optimum or even adequate performance. Further, the illumination must be highly uniform over an entire sub-field. Therefore, a cathode (emitter) of substantial size and high emission uniformity is required.
Further, the beam current in electron beam projection tools is relatively high and the electron beam column must be kept short to minimize effects of Coulomb interactions between particles of like charge as well as other, possibly more intractable physical constraints such as the maximum height of the tool/beam column relative to a standard clean room or requirements for other changes in beam column optics to obtain desired magnification and the like. Aberrations and deflection errors must be held to a small fraction of the minimum feature size to allow the sub-fields to be properly stitched together. In addition, to minimize variations of the critical feature dimensions across the subfield, the variation of current density distribution across the subfield must be held to 1% or less.
The elements of an electron optical system (e.g. lenses, deflectors and the like) are effectively defined by magnetic or electric fields which, in principle, have infinite extension at least along the axis of the system and, therefore, to a greater or lesser extent, interact or interfere with each other. Minimization of detrimental interference by separation of elements is usually very limited and may not even be feasible in adequate degree due to optical design constraints or environmental constraints on the size of the system. Complete suppression of such interference then requires increased complexity of the system and calibration/operation thereof due to added elements.
Further, an undesired interference may exist between an emitter of charged particles and an optical element in the vicinity of the emitter. For example, the field of a magnetic lens may affect electrons at their point of origin.
In summary, the number of magnetic and/or electrostatic elements and limited column length may cause interactions and interferences which are undesirable and which may not be feasible to adequately minimize.
It is therefore an object of the present invention to provide an arrangement and technique of reducing perturbation of low energy charged particles in the vicinity of their source in a charged particle beam tool.
It is another object of the invention to provide control of symmetrical and/or asymmetrical angular momentum and other perturbations due to interactions of the cathode, anode opening (or vice-versa for ion beam tools) and first condenser with low-energy charged particles in a charged particle beam tool.
It is a further object of the invention to provide a charged particle emitter having improved beam uniformity at a shaping aperture, reticle, wafer or object and reduced enlargement, astigmatism and distortion at the crossover in order to support enhanced performance of a charged particle beam tool such as an electron beam projection lithography tool to produce lithographic features.
In order to accomplish these and other objects of the invention, a charged particle beam illumination system is provided including a first condenser lens, an accelerating electrode, a charged particle emitter axially separated from the accelerating electrode, and an arrangement for reducing the magnetic field of said first condenser lens at said charged particle emitter.